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Pandas DataFrame to CSV

I want to append pandas data frames to the ends of CSV files. The tricky part is when I append rows, some of the times the columns may be different. I want code like this

a = pd.DataFrame([[1, 2]], columns= ["one", "two"])
with open("learn.csv", "w") as f:
    a.to_csv(f, header=True)

a = pd.DataFrame([[1, 2]], columns= ["one", "three"])
with open("learn.csv", "a") as f:
    a.to_csv(f)

to produce a CSV file that looks like this:

one, two, three
1, 2, None
1, None, 2

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Pandas DataFrame to CSV

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